A number of applications in the chemical arts require a substance which embodies, to the greatest extent possible, a high surface area per unit volume, typically measured in square meters per gram. These applications include, but are not limited to catalyst support, chromatography, chemical adsorption/absorption and mechanical adsorption/absorption. These applications generally require that a high degree of interaction between a liquid or gaseous phase and a solid phase; for instance, a catalyst support which requires that a maximum amout of reagents contact a catalyst in the quickest amount of time and within the smallest possible space, or a chromatagraphic technique wherein maximum separation is desired using a relatively small column.
More specifically regarding catalysts, heterogeneous catalytic reactions are widely used in chemical processes in the petroleum, petrochemical and chemical industries. Such reactions are commonly performed with the reactant(s) and product(s) in the fluid phase and the catalyst in the solid phase. In heterogeneous catalytic reactions, the reaction occurs at the interface between phases, i.e., the interface between the fluid phase of the reactant(s) and product(s) and the solid phase of the supported catalyst. Hence, the properties of the surface of a heterogeneous supported catalyst are significant factors in the effective use of that catalyst. Specifically, the surface area of the active catalyst, as supported, and the accessibility of that surface area to reactant chemisorption and product desorption are important. These factors affect the activity of the catalyst, i.e., the rate of conversion of reactants to products. The chemical purity of the catalyst and the catalyst support have an important effect on the selectivity of the catalyst, i.e., the degree to which the catalyst produces one product from among several products, and the life of the catalyst.
Generally catalytic activity is proportional to catalyst surface area. Therefore, high specific area is desirable. However, that surface area must be accessible to reactants and products as well as to heat flow. The chemisorption of a reactant by a catalyst surface is preceded by the diffusion of that reactant through the internal structure of the catalyst.
Since the active catalyst compounds are often supported on the internal structure of a support, the accessibility of the internal structure of a support material to reactant(s), product(s) and heat flow is important. Porosity and pore size distribution of the support structure are measures of that accessibility. Activated carbons and charcoals used as catalyst supports have surface areas of about 1000 square meters per gram and porosities of less than one milliliter per gram. However, much of this surface area and porosity, as much as 50%, and often more, is associated with micropores, i.e., pores with pore diameters of 2 nanometers or less. These pores can be inaccessible because of diffusion limitations. They are easily plugged and thereby deactivated. Thus, high porosity material where the pores are mainly in the mesopore (&gt;2 nanometers) or macropore (&gt;50 nanometers) ranges are most desirable.
It is also important that supported catalysts not fracture or attrit during use because such fragments may become entrained in the reaction stream and must then be separated from the reaction mixture. The cost of replacing attritted catalyst, the cost of separating it from the reaction mixture and the risk of contaminating the product are all burdens upon the process. In other processes, e.g. where the solid supported catalyst is filtered from the process stream and recycled to the reaction zone, the fines may plug the filters and disrupt the process.
It is also important that a catalyst, at the very least, minimize its contribution to the chemical contamination of reactant(s) and product(s). In the case of a catalyst support, this is even more important since the support is a potential source of contamination both to the catalyst it supports and to the chemical process. Further, some catalysts are particularly sensitive to contamination that can either promote unwanted competing reactions, i.e., affect its selectivity, or render the catalyst ineffective, i.e., "poison" it. Charcoal and commercial graphites or carbons made from petroleum residues usually contain trace amounts of sulfur or nitrogen as well as metals common to biological systems and may be undesirable for that reason.
Since the 1970s nanofibers have been identified as materials of interest for such applications. Carbon nanofibers exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces. Such vermicular carbon deposits have been observed almost since the advent of electron microscopy. A good early survey and reference is found in Baker and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14, 1978, p. 83, hereby incorporated by reference. See also, Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993), hereby incorporated by reference.
Nanofibers such as fibrils, bucky tubes and nanofibers are distinguishable from continuous carbon fibers commercially available as reinforcement materials. In contrast to nanofibers, which have, desirably large, but unavoidably finite aspect ratios, continuous carbon fibers have aspect ratios (L/D) of at least 10.sup.4 and often 10.sup.6 or more. The diameter of continuous fibers is also far larger than that of nanofibers, being always &gt;1.0.mu. and typically 5 to 7.mu..
Further details regarding the formation of carbon nanofiber aggregates may be found in the disclosure of Snyder et al., U.S. patent application Ser. No. 149,573, filed Jan. 28, 1988, and PCT Application No. US89/00322, filed Jan. 28, 1989 ("Carbon Fibrils") WO 89/07163, and Moy et al., U.S. patent application Ser. No. 413,837 filed Sep. 28, 1989 and PCT Application No. US90/05498, filed Sep. 27, 1990 ("Fibril Aggregates and Method of Making Same") WO 91/05089, all of which are assigned to the same assignee as the invention here and are hereby incorporated by reference.
While activated charcoals and other carbon-containing materials have been used as catalyst supports, none have heretofore had all of the requisite qualities of porosity and pore size distribution, resistance to attrition and purity for the conduct of a variety of organic chemical reactions.
Specifically, nanofiber mats, assemblages and aggregates have been previously produced to take advantage of the increased surface area per gram achieved using extremely thin diameter fibers. These structures are typically composed of a plurality of intertwined or intermeshed fibers.
The macroscopic morphology of the aggregate is controlled by the choice of catalyst support. Spherical supports grow nanofibers in all directions leading to the formation of bird nest aggregates. Combed yarn and open nest aggregates are prepared using supports having one or more readily cleavable planar surfaces, e.g., an iron or iron-containing metal catalyst particle deposited on a support material having one or more readily cleavable surfaces and a surface area of at least 1 square meters per gram.
Moy et al., U.S. application Ser. No. 08/469,430 entitled "Improved Methods and Catalysts for the Manufacture of Carbon Fibrils", filed Jun. 6, 1995, hereby incorporated by reference, describes nanofibers prepared as aggregates having various morphologies (as determined by scanning electron microscopy) in which they are randomly entangled with each other to form entangled balls of nanofibers resembling bird nests ("BN"); or as aggregates consisting of bundles of straight to slightly bent or kinked carbon nanofibers having substantially the same relative orientation, and having the appearance of combed yarn ("CY") e.g., the longitudinal axis of each nanofiber (despite individual bends or kinks) extends in the same direction as that of the surrounding nanofibers in the bundles; or, as, aggregates consisting of straight to slightly bent or kinked nanofibers which are loosely entangled with each other to form an "open net" ("ON") structure. In open net structures the degree of nanofiber entanglement is greater than observed in the combed yarn aggregates (in which the individual nanofibers have substantially the same relative orientation) but less than that of bird nests. CY and ON aggregates are more readily dispersed than BN making them useful in composite fabrication where uniform properties throughout the structure are desired.
Nanofibers and nanofiber aggregates and assemblages described above are generally required in relatively large amounts to perform catalyst support, chromatography, or other application requiring high surface area. These large amounts of nanofibers are disadvantageously costly and space intensive. Also disadvantageously, a certain amount of contamination of the reaction or chromatography stream, and attrition of the catalyst or chromatographic support, is likely given a large number of nanofibers.
Aerogels are high surface area porous structures or foams typically formed by supercritical drying a mixture containing a polymer, followed by pyrolysis. Although the structures have high surface areas, they are disadvantageous in that they exhibit poor mechanical integrity and therefore tend to easily break down to contaminate, for instance, chromatographic and reaction streams. Further, the surface area of aerogels, while relatively high, is largely in accessible, in part due to small pore size.
The subject matter of this application, deals with reducing the number of nanofibers needed to perform applications requiring high surface area by increasing the surface area of each nanofiber. The nanofibers of this application have an increased surface area, measured in m.sup.2 /g, as compared to nanofibers known in the art. Also advantageously, even assuming that a certain number of nanofibers per gram of nanofiber will be contaminant in a given application, the fact that less nanofibers are required for performing that application will thereby reduce nanofiber contamination.